A photographic process using silver halide has photographic properties, such as sensitivity, gradation, resolution, etc. which are superior to other means of photographic reproduction such as electrophotography, diazo photography, etc. Because of these superior properties, it has been the most prevailing type of photographic process. Recently, various techniques for simply and rapidly producing an image have been developed. These techniques involve changing the method for processing latent images formed in photographic materials of silver halide-using type from the conventional wet developing method using a developing solution or the like to the dry developing method using a heating means or the like.
The most successful examples in the art of photographic materials of the kind which can produce photographic images using such a dry processing method as described above are at present heat-developable photographic materials such as those described in U.S. Pat. Nos. 3,152,904 and 3,457,075. These heat-developable photographic materials utilize compositions which contain, as essential components, an organic silver salt, a catalytically small amount of photocatalyst (e.g., silver halide) and a reducing agent, respectively. These photographic materials are stable at ordinary temperatures. However, when heated to a temperature of about 80.degree. C. or higher, and preferably 100.degree. C. or higher, after imagewise exposure, the photographic material produces silver through a redox reaction of the organic silver salt, which can act as an oxidizing agent, with the reducing agent which takes place in its light-sensitive layer by the catalytic action of the exposed photocatalyst present in the neighborhood of the above-described agents. The silver which is produced in the exposed area of the light-sensitive layer is rapidly blackened to produce a contrast with respect to the unexposed area (background), i.e., to result in the formation of an image.
However, conventional heat-developable photographic materials are not particularly desirable due to the generation of silver which can be seen in white background areas which should be image-free, that is, fog is generated.
The most effective means for reducing the fog, though it is almost impossible to completely prevent the occurrence of fog, is at present, to use the mercury compounds as described in Japanese Patent Publication No. 11113/72 (corresponding to U.S. Pat. No. 3,589,903).
As well known in the art, however, mercury compounds are virulently poisonous, and cannot be employed due to the environmental pollution which is created because, for example, mercury is transpired when the photographic material is submitted to heat-development and, further, the mercury is liable to effuse from the photographic material if it is scrapped after use.
In Japanese Patent Application (OPI) Nos. 119624/75, 120328/75, 70543/81 and 99335/81, etc. there are described that certain organic polyhalogeno compounds are useful as antifoggants for heat-developable photographic materials. However, the antifogging effect obtained with these compounds is much lower than that which is obtained using mercury compounds.